The present invention relates to the inflation of tires with oxygen-depleted air and to a method for varying the composition of a gas mixture for tire inflation.
It is known that tires, once fitted or refitted onto a wheel, are usually inflated by compressed air. Air inflation proper can occur either on the tire changing machine or within a receiving and protection cage for safety reasons. The compressed air used is usually fed through a supply line and suitably filtered. Inflating devices typically comprise an intake valve, an outlet valve and a pressure gauge for detecting the internal pressure of the tire.
Oxygen is present in air in a percentage of approximately 20% and is responsible for several undesirable effects. First of all, the O2 molecules tend to escape from the tire by diffusion, and thus the inflation pressure will be eventually unstable. Accordingly, the trend is to use tires inflated below their optimum value or threshold inflation range, which causes faster tire wear and affects safety in driving of the vehicle. Secondly, the presence of oxygen, whose percentage increases in the presence of moisture, is responsible for oxidation of metallic components (wheelrim) that form a tire-wheel, thus causing faster aging thereof. Finally, oxygen, e.g. at concentrations higher than 12% and in the presence of heat, can cause explosions and thus fire.
To obviate these drawbacks, it has already been suggested to inflate tires with nitrogen alone, e.g. in racing vehicles and in aeronautics, but inflation with nitrogen entails not only nitrogen supply problems but also the use of bulky devices which as such are scarcely practical for daily use besides being highly expensive. As a matter of fact, nitrogen is usually marketed as a gas in large steel cylinders at high pressure (about 200 bar) or in liquid state. It is accordingly necessary to provide, at the site of use, a pressure reduction unit in the first case and an evaporator in the second case, and inflation with nitrogen always occurs after a pre-inflation with air performed during fitting of the tire on a tire changing machine.
Once it has been inflated, a tire is in every respect a thermodynamically open system which is subject to variations as to mass (m) and its state (p, T), since the tire behaves substantially like an osmotic membrane, i.e. exchange occurs between the environment inside the tire and the outside environment (atmosphere). Regardless of the type of gas mixture used to inflate a tire, migration of oxygen (O2) occurs through the tire (in one direction or the other). The oxygen that passes through the mass of rubber-like material, besides damaging the structure of the tire (aging), also causes an unwanted variation in the inflation pressure originally generated during tire inflation.
When inflation is performed with atmospheric air (a gas mixture formed roughly by 20% O2, 78% N2 and 1% Ar and CO2), for example, at a pressure p=3 bar and at an ambient temperature T=299xc2x0 K., owing to the different partial pressures of the various gaseous components of the mixture (which generally increase as the overall inflation pressure increases) and the permeability of the rubber-like material that constitutes the tire, there occurs a slow migration of O2 molecules from the inside toward the outside of the tire, which reduces the overall inflation pressure. As is known, this leads to less safe driving and to higher wear of the tire tread.
If instead inflation is performed with oxygen-depleted air, for example with N2, temperature and inflation pressure being equal, a reverse migration of O2 from the outside to the inside of the tire occurs eventually, thereby increasing the pressure with respect to the initial inflation pressure. In general, the lower the pressure difference between the inside and the outside of the tire, the more conspicuous is this effect.
The main object of the present invention is to eliminate or drastically reduce the above listed drawbacks and in particular those drawbacks related to the variation in inflation pressure caused by migration Of O2 through the rubber material of the tire. More particularly, the aim is to take advantage of the phenomenon arising from the behavior of an oxygen-depleted inflation mixture, and since nitrogen passes through rubber at a slower rate than compressed air (of which approximately 20% is O2), a tire inflated with nitrogen requires less frequent pressure checking and explosion is avoided.
According to a first aspect of the present invention, a tire inflation system is provided which has a compressor and at least one inflating nozzle or gun designed to be connected to a respective inlet for a tire and to control the fluid flow supplied thereto, and is characterized in that it comprises a source of oxygen-depleted air which is arranged upstream of said compressor and downstream of said nozzle or nozzles.
According to another aspect of the present invention, there is provided an inflation method which comprises varying the composition of air used for inflating a tire depending upon the inflation pressure recommended for a given tire.
More particularly, the method according to the present invention comprises decreasing the percentage of oxygen (O2) as the inflation pressure required for the tire rises.
According to another aspect of the present invention, a device is provided for program-controlled variation in the composition of a gaseous mixture for inflating a tire, which includes a first source of at least one main gas, a second source of at least one additional gas, at least one compressor suitable for compressing to a preset pressure a gaseous mixture of at least said main gas and an additional gas, at least one dispenser of compressed mixture which is connected to the delivery of said compressor, and is characterized in that it comprises at least one mixer for said at least one main gas and said at least one additional gas which is suitable for dispensing a gaseous mixture having a desired composition, a gas analyzer means which is arranged downstream of said mixer, valve means for controlling flow between said mixer and said first and second gas sources, and program control means arranged to control said valve means in response to control signals from said analyzer means.